Control of the power factor of switched mode power supplies is becoming increasingly important, particularly for SMPS which are designed to be applicable directly from a mains input power. This is in part due to legislation such as European Union regulation EN 61000-3-2, and in part due to increasing emphasis on energy efficiency, such as required by the 80+ initiative.
In order to ensure that mains harmonic levels are adequately low, currently SMPS use either a passive filter or an active circuit to effect power factor correction. For low to medium power applications (that is, below a few kilowatts) power factor correction is achieved by using a boost converter, fly-back converter or Buck derived converter (for example a forward converter) with appropriate control of the active switch. There are two basic controller schema: the first uses fixed frequency in either discontinuous conduction mode or continuous conduction mode (DCM and CCM respectively), depending on the power level; the second uses boundary conduction mode (BCM) operation. Although there are no rigid boundaries between the two schema, many high-volume applications operating in the 100 W range such as notebook computers, DVD players, pre-conditioners in desktop PC supplies such as the Silverbox, and the like, use BCM operation.
One drawback of BCM operation is the high switching frequency which can occur under light loads. In order to prevent undesirably high switching frequencies, a method of operation which includes valley skipping in conjunction with BCM has been developed. This method is disclosed in U.S. Pat. No. 6,256,210B1, the entire contents of which are incorporated herein by reference. By modifying the BCM such that valleys are “skipped”, at the end of the secondary stroke before the switch is closed to restart the primary stroke, an upper limit of switching frequency is achieved. Thus, instead of traditional boundary conduction mode, in which the switch is closed immediately on the inductor current returning to zero, valley skipping involves retarding the switch closure. When the current has returned to zero, a resonance of the voltage at the switching node starts through the inductor and parasitic capacitance, the parasitic capacitance being mainly in the switch. This results in a ringing around zero inductor current. In the valley skipping control, the switch is not closed until the inductor current is at a subsequent zero-crossing (which, for a valley at the switching node, is positive-going). Optionally, the number of valleys skipped can be counted, or alternative means used to determine at which zero-crossing the switch is to be closed. Introducing the ringing into the switching cycle increases the overall switching cycle period, and thus reduces the switching cycle frequency. As the power level is reduced further, corresponding to even lighter loads, the ringing time can be further increased, maintaining the same on-time for the switch. (Since the switch on-time determines the energy transferred per switching cycle, the latter remains constant; however, increasing the ringing period increases the switching cycle period such that there are fewer switching cycles per unit time, so the power transferred—which is the energy transferred per unit time—is decreased.)
By introducing valley skipping into BCM conduction, perturbations are introduced and the power factor is reduced: each time the frequency clamping increases the number of skipped valleys, a sharp step can be found in the averaged inductor current (which in the case of a boost converter is the line current). This is illustrated in FIG. 1, which shows inductor current 11 (averaged over the switching cycle) over a mains half-cycle. As the average current decreases, each time the frequency clamp forces the controller to skip an additional valley, there is a noticeable step 12, 12′ in the average current.
FIG. 2 shows the waveform of the average current 21, in front of the necessary EMC filter, over the same mains half-cycle. Distortion 22, 22′ of average current from of the desired sine wave is still clearly visible.
Thus there is a need for an improved method of operating a BCM controller such that the input current follows more closely a perfect sine wave, leading to an improved power factor which is closer to unity.